I run 4-5 PSI in them. That is the best PSI in my opinion. Lots of tread on the ground.
About 80% of my riding is 40MPH+ on logging roads doing alot of drifting and sliding around and I run my big horns 4-5PSI with 0 problems.I would agree to drop it down from 15psi. They were probably inflated that high to seal the bead on the rim. Just remember the lower the pressure the more chance you stand of breaking the tire off the rim when riding aggressively or in off-camber situations.